Various optical systems, such as optical pick-up apparatuses for recording and reproducing information on optical information recording media such as CDs (compact disks), have been known. In addition to that, DVDs (digital video disks), which are similar in terms of size to conventional CDs and enhanced in terms of capacity, have been developed recently.
In comparing a CD and a DVD, an infrared semiconductor laser having an optical wavelength of substantially 0.78 μm is used for a CD, while a red semiconductor laser having an optical wavelength of 0.63 μm to 0.65 μm is used for a DVD. In a DVD, a numerical aperture (hereinafter, abbreviated as NA) of an objective lens of an optical pick-up is set to 0.6, and the thickness of its transparent substrate is set to 0.6 mm, or half of a conventional CD. Furthermore, a DVD has a track pitch of 0.74 μm and a shortest pit length of 0.4 μm. A CD has a track pitch of 1.6 μm and a shortest pit length of 0.83 μm, that is, about as twice those of a DVD. Namely, a DVD has higher density than a CD.
As mentioned above, various optical disks different from each other in the substrate thickness, the recording density, and the working wavelength are on the market, and thus objective lenses that can correspond to various optical disks have been proposed. For example, JP 10(1998)-143905 A describes an optical head apparatus that divides a light beam converged by an objective lens into a plurality of light beams, optimizing a light beam at a ring-shaped portion with respect to a CD, while forming a spot with a light beam positioned other than the ring-shaped portion and optimizing with respect to a DVD. The above-mentioned optical head apparatus is described further by referring to FIGS. 12 and 13.
FIG. 12 shows a conventional optical head apparatus and an optical information recording/reproducing apparatus. The optical head apparatus shown in this figure is substantially as same as an optical head apparatus described in JP 10(1998)-143905 A. FIG. 13A is a cross-sectional view showing an objective lens shown in FIG. 12 and also a light beam passing through a DVD as an optical disk. FIG. 13B is a cross-sectional view showing an objective lens shown in FIG. 12 and also a light beam passing through a CD as an optical disk.
A structure of a conventional optical head apparatus and an optical system will be described below, referring to FIG. 12. For example, when playing an optical disk 7 as a DVD, a light beam 2 having a wavelength of 650 nm is emitted from a semiconductor laser 1. The light beam 2 passes through a beam splitter 33, a collimator lens 3 and a beam splitter 5 in this order, and it is converged on the optical disk 7 by an objective lens 20. The light beam reflected by the optical disk 7 was reflected by the beam splitter 5 and received by a photodetector 4. The photodetector 4 has a photoreceptor, and thus the reflected light received by the photodetector 4 is converted into an electric signal.
When playing a CD as an optical disk 7, a light beam 32 having a wavelength of 780 nm is emitted from a semiconductor laser 31. The light beam 32 is reflected by the beam splitter 33, and converged on the optical disk 7 by the objective lens 20 in the same manner as the case of the light beam 2 used for playing a DVD. The light beam 32 reflected by the optical disk 7 is reflected further by the beam splitter 5, received by the photodetector 4 and converted into an electric signal.
Furthermore, a disk discrimination circuit 23 for discriminating the type of the optical disk 7 and a reproduction signal circuit 24 for selectively reading information of the electric signal corresponding to the type of the optical disk 7 are connected to the optical head apparatus shown in FIG. 12, thereby configuring an optical information recording/reproducing apparatus. In FIG. 12, 6 denotes a mirror for directing a light beam upwards, 8 denotes a detection lens, 9 denotes an astigmatism generator, 21 denotes an actuator for adjusting focuses of the objective lens, and 22 denotes an actuator (ACT) driving circuit.
The objective lens 20 shown in FIG. 12 has a characteristic portion on at least either of an incident plane or an emission plane. Specifically, as shown in FIGS. 13A and 13B, the objective lens is provided with an intermediate region A2 formed on the incident plane for a light beam. The intermediate region A2 is shaped like a doughnut or a ring having an outer diameter smaller than the entire effective diameter with respect to an optical path region. The interior of the intermediate region A2 makes a center region A1, and the exterior of the intermediate region A2 makes a peripheral region A3.
As shown in FIG. 13A, the center region A1 and the peripheral region A3 have optimized curvatures respectively, and thus when a light beam 2 having a wavelength of 650 nm enters, focusing is obtained on a recording surface of a DVD covered with a transparent substrate 7a. As shown in FIG. 13B, the intermediate region A2 has an optimized curvature, and thus when a light beam 32 having a wavelength of 780 nm enters, focusing is obtained on a recording surface of a CD covered with a transparent substrate 7b that is thicker than the transparent substrate 7a. 
The intermediate region A2 is provided to form stepped portions between the intermediate region A2 and the center region A1, and also between the intermediate region A2 and the peripheral region A3. Therefore, an advance or a delay is generated in a phase of a light beam passing through the intermediate region A2 with respect to phases of light beams passing through the center region A1 and the peripheral region A3, thereby suppressing generation of a wavefront aberration on the recording surface of the disk 7.
As mentioned above, the objective lens 20 as shown in FIG. 13A or 13B can be used alone for both a DVD and a CD. Arrows in FIGS. 13A and 13B indicate traveling directions of light beams reflected on the recording surfaces of the optical disks. D1 in FIG. 13A indicates a light beam reflected on a recording surface of a DVD, and D2 in FIG. 13B indicates a light beam reflected on a recording surface of a CD.
However, such a conventional optical head apparatus causes a problem as shown in FIG. 13A. Namely, a light beam passing through the intermediate region A2 of the objective lens 20 does not contribute to recording or reproducing during a recording and a playback of a DVD, and thus an efficiency of a laser beam emitted from the semiconductor laser 1 is low.
For a conventional optical head apparatus, a spherical aberration will be generated on a recording surface of a CD due to a light beam passing through the center region A1 of the objective lens 20 during a playback from the CD. As a result, the reproduction signal will deteriorate since the spherical aberration is not subjected to any substantial correction.
An object of the present invention is to provide an objective lens, an optical head apparatus and an optical information recording/reproducing apparatus, which can serve to improve optical efficiency during a playback of an optical information recording medium having a thin transparent substrate, and also suppress deterioration in a reproduction signal during a playback of an optical information recording medium having a thick transparent substrate.